Long-period fiber grating humidity sensor based on spider silks

We propose a long-period fiber grating (LPFG) humidity sensor based on two kinds of spider silks. Both spider dragline silk (SpDS) and spider egg-case silk (SpECS) for sensor preparation are produced by the Araneus ventricosus. SpDS can contract up to half the original length in the axial direction when the ambient relative humidity (RH) increases due to its supercontraction property. However, the length of SpECS axial contraction is much smaller than that of SpDS as RH increases, because SpECS does not have the supercontraction property like SpDS. We fabricated a LPFG by irradiating a single-mode fiber with a high-frequency CO 2 laser. One end of LPFG is fixed by ultraviolet glue, and the other end is inserted into the quartz round tube without fixing. Therefore, the unfixed end of LPFG serves as the movable end to ensure that the direction of movement of LPFG is controllable. In addition, SpDS and SpECS exert pull forces on LPFG in two opposite directions, and the overall structure forms an X shape. When the ambient RH increases, the length of SpDS becomes shorter due to the supercontraction property. It changes the structure curvature of LPFG, leading to the transmission spectrum shift. When the RH decreases, SpECS provides a recovery pull, like a spring, that restores SpDS to a taut state when it relaxes. The curvature of LPFG is reduced, and the transmission spectrum shifts in the opposite direction. Therefore, we monitor the RH by observing the wavelength shift of the transmission spectrum. Our sensor achieves humidity detection with − 0.2039 nm/%RH sensitivity in the 50%RH-80%RH range. Additionally, we can adjust the amount and length of the two spider silks to increase the maximum detection humidity range closer to 100% RH at the expense of reduced sensitivity. The proposed humidity sensor has the advantages of reversibility, good repeatability, and environmental friendliness. We propose a long-period fiber grating (LPFG) humidity sensor based on two kinds of spider silks. The supercontraction property of spider dragline silk (SpDS) enables it to axially contract up to half its original length with increasing ambient relative humidity (RH). In contrast, the length of spider egg-case silk (SpECS) hardly varies with the change of RH. When the ambient relative humidity increases, the shortening of the SpDS length changes the structure curvature of LPFG, which leads to the shift of the transmission spectrum. When the RH decreases, SpECS will reset the optical fiber, and the transmission spectrum will shift in the opposite direction. Therefore, the humidity sensor achieves a linear response in 50%RH-80%RH with a sensitivity of − 0.2039 nm/%RH. The sensitivity and detection range of the sensor can be adjusted by changing the number and length of spider silk according to the demand, and the maximum detection humidity range can be increased closer to 100% RH. The LPFG humidity sensor based on spider silk can play a positive role in environmental friendliness and physiological monitoring. • This manuscript proposes and experimentally demonstrates a novel long-period fiber grating humidity sensor based on two kinds of spider silks. • The sensor achieves a linear response in 50%RH - 80%RH with a sensitivity of − 0.2039 nm/%RH. • The sensitivity and detection range of the sensor can be adjusted by changing the number and length of spider silk according to the demand.